1. Field of the Invention
This invention relates to image forming optical systems, and more particularly, to systems that employ catalog (stock) lenses and that can employ field correcting lenses.
2. Description of Related Art
A. Use of Catalog Achromatic Doublet Lenses
It has long been known that chromatic aberration in lenses can be corrected by combining two lens elements of different glass types, one of positive power and one of negative power, into a single lens assembly. If the combination is to have a positive focal length, the positive element is made of a low dispersion glass commonly referred to as “crown” while the negative element is made of a high dispersion glass commonly referred to as “flint”. When the glass types and relative powers of the two elements are correctly chosen, the resulting lens has a focal length that is largely independent of wavelength over a substantial working wavelength range. These lenses are known as achromatic doublets.
The most commonly used form of achromatic doublet is also substantially corrected for spherical aberration for an object at infinity, and is then often referred to as a “telescope objective”. In addition, an telescope objective can also be substantially corrected for coma with the aperture stop at the lens; these variants are called “aplanats”. (Hereinafter, I always use the term “aplanat” to refer to a lens which is corrected for both spherical aberration and for coma with the stop at the lens. I will use the term “aplanatic” in a more general way, as will be discussed below.)
Today such lenses have a much wider range of applications than telescopes, and achromatic doublets are widely available from a large number of manufacturers who publish catalogs of them. The two elements of these catalog achromatic doublets almost always have a common radius of curvature on their adjacent surfaces and are cemented together at these surfaces to lower the cost. Cemented doublets sacrifice some ability to correct higher order aberrations, but with modern glass types, their correction for axial color, (third order) spherical aberration and (third order) coma can be quite good.
The resulting lenses, which I will refer to hereinafter as standard achromats, are very useful since the correction of the monochromatic aberrations allow them to perform much more like ideal lenses than do the simple singlet lenses that are also commonly available. As a result, standard achromats are widely used in laboratory, prototype, and small production run optical instrument applications where the cost and/or delivery time of custom designed and/or manufactured lenses cannot be justified.
It is known to those skilled in the art that although standard achromats are usually corrected for coma, the size of the field of view over which they provide good performance is limited by the additional aberrations field curvature and astigmatism. The useful field of view of these lenses is usually taken to be 2 degrees radius or less. In fact, it is known from optical aberration theory that if both spherical aberration and coma are corrected in a thin lens, that the astigmatism will not affected by the position of the aperture stop, so that an aplanat will always have an uncorrected astigmatism when the lens is used in its normal orientation.
Note that hereinafter I will sometimes refer to “field aberrations” or “a field aberration”. In this document I use the term “field aberration” as a generic term that may include any or all of coma, field curvature, astigmatism, or lateral chromatic aberration.
It is also known to those skilled in the art that if one reverses the orientation of a standard achromat (so that the lens is no longer corrected for spherical aberration), then one can find a position of the stop that minimizes the astigmatism of the lens. With the stop in this position, the lens can be used over a much larger field of view; a field angle in excess of 20 degrees is not unreasonable. The problem is that in this case the aperture of the lens must be kept small, since the large spherical aberration produced by the reversed lens will dominate the quality of the image otherwise.
There are many practical applications in which lenses are used in combination to relay and otherwise manipulate images in which the field of view of standard achromats, when used at large aperture, is not adequate. In addition, the focal ratio of achromats is often larger than needed to obtain the necessary amount of light in the image.
B. Use of Stock Multi-element Lenses
While there are a number of existing multi-element photographic and video lenses immediately available from stock that have low focal ratios and good performance over wide fields of view, these lenses cannot often be employed in relay and finite conjugate applications that arise in optical instrument design. One of the recurring frustrations that optical system designers face these days is that the manufacturers of these lenses generally refuse to release their detailed designs to ease their use within optical systems. In fact, the problem is more than lack of access to their detailed designs; additional problems with making use of existing multi-element, lenses in these applications are:                (a) Accurate paraxial properties are not made available by their manufacturers, so that it is not possible to accurately determine the layout of an optical system that will perform a desired function without first laboriously characterizing the lenses.        (b) Locations of the entrance and exit pupils are not made available by their manufacturers, so that one cannot determine the vignetting that will occur when using such lenses in combination with other lenses, once again without performing a laborious characterization.        (c) The internal stops inherent in these lenses often lead to unacceptably large vignetting when these lenses are combined into relays.        (d) These lenses are often physically too large to fit into the available space. They lack the flexibility to address the wide variety of system mechanical constraints that appear in practice.        (e) These lenses are often too expensive for the application to bear.        
As a result, individuals relaying or manipulating images within optical instruments often make use of standard achromats for applications requiring a wider field of view than achromats can handle and accept the resulting poor performance because there is simply no better alternative short of custom made lenses.
C. Use of Correcting Lenses
It is a well established principle in lens design to use multiple lenses in cooperation so that one lens or lens group can correct for aberrations caused by another lens or lens group. There are several known approaches to correcting field aberrations of a lens, some of which have previously been applied to achromats. However, none of the high quality approaches have, to my knowledge, been conceived of as optical components of general utility. As I will demonstrate, none of the prior art approaches addresses all aspects of the current problem, and in fact, the prior art strongly teaches away from the solution that I have devised.
C1. Field Flattening Lenses
In the 1870's, Scottish astronomer C. Piazzi-Smyth suggested placing a lens element having a strong negative power close to the image plane for correcting the field curvature in an image. In concept, the aberrations of the field flattening lens have little effect on the image, and the size and location of the image are also little changed by its presence. Unfortunately, in practice, only a small amount of field curvature can be corrected in this manner before the aberrations of a single-element field flattener do affect the quality of the image. While sometimes useful, in no way can single-element field flatteners be considered a general purpose solution to improving the performance of achromat-based systems.
Two-element field flatteners are also known, for example Friedman, U.S. Pat. No. 4,772,107, and Sugawara, U.S. Pat. No. 6,563,642. Just as with the Smyth field flattener, these act to increase the focal length of the primary optical system. In the examples disclosed by Friedman the magnifications (that is, the ratio of the focal length of the lens combination to the focal length of the primary lens) are 1.08 and 1.09, while in the case of Sugawara, the magnifications of the field flatteners shown are about 1.4. While almost everyone would agree that Friedman's lenses are field flatteners, I believe that there would be disagreement among those skilled in the art as to whether Sugawara's lenses should be considered to be field flatteners, as he refers to them, or whether these are really the diverging element in what amounts to a telephoto system. While telephoto systems have their uses, they do not address the problem I am concerned with because the focal ratio of the combination is actually larger than the focal ratio of the primary lens. What is needed is that the focal ratio of the system of achromat plus corrector should be smaller than the focal ratio of the achromat used alone, thereby providing, at least potentially, a higher irradiance (“more light”) in the image.
C2. Field Correcting Lenses
Reflecting telescopes often exhibit coma. It is common to use multi-element coma correcting lenses with large professional telescopes to increase their useable fields of view, as discussed, for instance, by C. G. Wynne, in the journal article “Field correctors for large telescopes”, Applied Optics, vol. 4, No. 9, September 1965, pp. 1185–1192, and by R. N. Wilson in “Corrector systems for Cassegrain telescopes”, Applied Optics, vol. 7, No. 2, February 1968, pp. 253–263. Depending on the design of the telescope, such correctors may be intended to correct for coma only or for astigmatism and field curvature or for all three aberrations simultaneously. However, the fields of view over which such corrections are offered are small; in fact the widened fields for these telescopes are often smaller than the unaided field capability of an achromat. In addition, the color correction required in such correctors is such that it can often be satisfied by making all elements of the corrector from the same glass; the same is not true for the optical instrument applications I am addressing. Finally, these correctors have essentially no effect on the speed of the cone illuminating the image, which is an advantage for their intended applications, but is not what is needed here.
C3. Field Compressor/Corrector Lenses
In amateur astronomy, it is now common to use a multi-purpose lens near the focal plane of a Schmidt-Cassegrain telescope when photographing the heavens using a solid-state imager. This lens can correct for coma and for field curvature of the telescope, and it also reduces the focal ratio of the cone illuminating the image. An example is provided by H. Rutten and M. vanVenrooij in the book Telescope Optics, Willman-Bell, © 1988–2002, pp. 157–159. Similar lenses have also been provided for use with Newtonian telescopes, an example being U.S. Pat. No. 4,881,801 by R. J. Gebelein. I call these multi-purpose lenses “field compressor/corrector” lenses. Other terms that are sometimes used are “focal reducers” and “telecompressors”, but I prefer to reserve these terms for lenses that do not combine field correction with focal ratio compression.
Field compressor/corrector lenses could conceivably address the problem I am trying to solve. However, field compressor/corrector lenses designed for use with reflecting telescopes are not directly applicable to use with achromats because the field aberrations of an achromat are not the same as are the field aberrations of a reflecting telescope. There have been provided focal reducers for achromatic (indeed for apochromatic, that is, having a color correction superior to that of an achromat) refractive telescopes which reduce the focal ratio of the telescope at the image without destroying the already excellent image quality provided by the primary lens. These focal reducing lenses are superb for their purpose, but they are multi-element, expensive, and are designed to work only over small field of view. They also are not designed to correct for field aberrations. Note that the designs of multi-element field compressor/corrector and focal reducer lenses offered commercially for use in amateur astronomy are, for the most part, proprietary and have not been published.
There have been suggested, and even provided, focal reducing lenses for use with refracting telescopes in amateur astronomy. However, these lenses are not expected to produce high quality images. This is clear when one considers that according to their manufacture's literature they are considered to be equally applicable to use with either reflecting telescopes or refracting telescopes, and that no construction or performance data is ever provided. An example, I believe, is the “2.5×CCD Compressor” sold by Lumicon International of Simi Valley, Calif. It is important that the user be enabled to provide the best possible performance in his or her application and this certainly is not the case with these lenses. In fact, recently the use of a standard achromat has also been suggested for use as telescope focal reducer, but it is clear to anyone skilled in the art that such a lens can only provide modest performance at best. In summary, in the prior art a doublet field reducer is used on a refractive telescope either by those who are not skilled in the art, or by those who are not making a serious attempt to correct aberrations of the telescope objective.
C4. Meniscus Correctors
Thick meniscus lenses have also been used as correctors in otherwise reflective telescope systems, to form what are referred to as “catadioptric” telescopes. However, it was pointed out by T. H. Jamieson in the journal article “Thick meniscus field correctors”, Applied Optics, vol. 21, No. 15, 1 Aug. 1982, pp. 2799–2803, that a thick meniscus lens can also act as a field corrector for an achromatic doublet. He also points out that a thick meniscus can have considerable positive power while offering field correction, that is, that it can act as a field compressor/corrector. Jamieson depicts examples and discusses the use of these lenses in a general way, but what this reference lacks is any consideration of a corrector as a standard optical component of general utility; it is instead considered as a component to be used inside a more complex lens. It also lacks any consideration of color correction, which makes sense, since when the meniscus is used inside a more complex lens, the color correction can be accomplished elsewhere. In fact, it is clear that the meniscus correctors discussed by Jamieson, even though he refers to them as “thick”, are much too thin to be color corrected. On the contrary, I have found color correction to be extremely important in addressing the current problem.
Much thicker examples of meniscus lenses used near an image were disclosed by F. E. Altman in U.S. Pat. No. 3,014,407. The lenses disclosed by Altman are used in pairs as pupil relay lenses. On analysis, one finds that Altman's lenses make very poor field correctors, and also that they are not reducers but instead act to increase the focal ratio at the image. Thus, Altman's lenses are not a solution either; these lenses are useful only in relay pairs where one of them can correct for the aberrations of another. Altman does disclose the use of a thick meniscus formed from two different glasses, i.e., a doublet. However, it becomes clear on analysis that this doublet does not result from any thought of color correction; Altman used it, as he discusses, because an aspherical surface is used on one surface of the lens. Making the lens as two components then allows use of a thinner piece of glass that is more easily slumped or molded to have the aspheric surface shape; this piece is then cemented to a second piece of glass to make up the complete meniscus element at the desired large thickness. In fact, the performance of Altman's lenses as field correctors are so poor that color correcting them would be irrelevant. Altman heavily relied on the fact that when these lenses are used in pairs for a 1:1 pupil relay the lateral chromatic aberration of one cancels the other.
D. Lens Combinations
In the prior art there are known combinations of two component lenses and in many of these combinations one component could be considered to be correcting a field aberration of the other component. However these combinations have, for the most part, been considered useful only as combinations, that is, their components have not been considered to be useful individually. Lens combinations per se, therefore, cannot be considered as prior art to a field correction lens of general utility, as there is no guidance as to how or when the “corrector” lens of the pair could be used to correct aberrations any lens other than the one with which it was originally combined.
That being said, it is important to examine prior art lens combinations because of the teachings about what is possible in the way of correction of field aberrations contained in the prior art. In the few cases where a first component of the combination could be considered useful when used by itself, the second component has generally been considered unable to correct field aberrations of the first component. There are strong statements to this effect in the prior art, and most of the known lens combinations have designs that are consistent with these statements. In those few examples I have found where the designs are not consistent with these teachings, the resulting low performance of the combinations is exactly what one would expect according to these teachings. The issues here are complex, and a more detailed discussion is deferred until Section 12 of the Detailed Description of the Invention.
E. Summary
Standard achromats are high quality lenses, immediately available from stock, that are widely used in laboratory and short production run applications where the cost and/or delivery time of custom made lenses is unacceptable. Their optical performance is adequate where the application requires imaging only over narrow fields of view, or where the application requires only a large focal ratio. However, there are many applications where the performance of standard achromats is not adequate, yet they are used simply because there is currently no better available option.
Existing stock multi-element lenses are often not helpful in these applications, either because necessary data are unavailable or because they lack the mechanical and/or optical flexibility required.
Field compressor/corrector lenses might be considered suitable for combining with achromats to improve the performance of achromats in these applications, however those previously known are not suitable for one reason or another. High quality correctors have been implemented as multi-element lenses, and doublet field compressor/correctors have been used only when optical quality was not a serious goal. In fact, the prior art in two-component lens combinations teaches strongly against trying to combine an achromat with a doublet compressor/corrector lens if a high quality image over an extended field of view is required. The use of thick meniscus correctors has also been suggested, but the correction of color aberrations in the corrector was not then considered necessary.
Fundamentally, what is needed is an optical system which can produce an improved performance in optical systems that must operate at larger fields of view and/or smaller focal ratios than can be handled by standard achromats. The new optical system should be as easy to apply and as flexible as existing standard achromats, both optically and mechanically. In addition, the new system should have a cost in line with the cost of standard achromats. Ideally, this solution would be suitable in all respects for selling in catalogs just as are standard achromats. The solution should be as universal as possible in that it should have a range of applicability approaching that of achromats themselves. Last, but not least, the new optical system should be supplied with all of the information needed for it to be effectively applied to a user's requirements, no matter what those requirements happen to be.